The present invention relates to a preamplifier for amplifying a small signal from a transducer having an inductive signal source impedance with an improved signal-to-noise ratio S/N of the signal demodulated from the amplified signal and more particularly a preamplifier especially adapted for use in a system for recording and reproducing angularly modulated television signals.
A preamplifier in a home type video tape recorder consists of an amplifier with an input terminal grounded through a parallel circuit of a resistor and a capacitor connected to a magnetic head. The inductance of the magnetic head and the capacitor form a parallel resonant circuit, and the resistor serves as a damping resistor of resonant circuit. The gain of the amplifier is maximum at the resonant frequency fr of the resonance circuit.
The demands upon the performance of this preamplifier will become apparent from the following description of the process for recording and reproducing the television signal. When the sinusoidal waveforms recorded on the magnetic tape and reproduced across the magnetic head, the maximum output voltage is obtained at the frequency fo (approximately 1 MHz or less) which is dependent upon the gap width of the magnetic head and the relative velocity between the magnetic tape and head.
Since the bandwidth of the video signal of the television signal is 3 MHz, the bandwidth of 3 MHz is also required in the frequency modulation of the video signal when the latter is recorded. Therefore the resonant frequency of the resonant circuit of the preamplifier is set at 5 MHz so that the output voltage-frequency characteristic curves of the preamplifier may be made flat. In general, the upper and lower limits of the bandwidth in the frequency modulation of the video signal are 4.5 MHz and 3.5 MHz, respectively, and the frequency modulation is made in such a way that the sync clip level may have a low frequency while the white level may have a high frequency. The carrier and the lower sideband are recorded on the magnetic tape, but the upper sideband is only partially recorded. When the video signal is reproduced with a video tape recorder with a narrow bandwidth, a peculiar phenomenon called "reversal" tends to result. In order to avoid this phenomenon, the resonant frequency fr is set higher than the upper limit (4.5 MHz).
As a result of ever improving magnetic recording and reproducing techniques, information or data may now be recorded on magnetic tape at an extremely high density. Consequently, while by the EIAJ (Electronic Industrial Association of Japan) standards, the relative velocity between the magnetic head and tape and the track width were 11 m/sec and 120 .mu.m respectively, the standards for recently developed devices are 6 m/sec and 30 .mu.m, respectively. This means that the recording density per unit area on the magnetic tape has been increased by 8 times as high as before. However, the slower the relative velocity and the shorter the track width, the lower the signal voltage reproduced across the magnetic head becomes. Even though the decrease in output voltage has been avoided by the improvements of the magnetic tapes and heads to some extent, it results in serious degradation of the signal-to-noise ratio S/N as will be described below.
First, in order to obtain a high S/N ratio with a low output voltage, noise must be suppressed as much as possible. However, the causes and nature of noise produced in a system consisting of a magnetic tape, a magnetic head and a preamplifier had not been thoroughly investigated and analyzed. Furthermore the impedance of the signal source including the magnetic head varies in response to the frequency. As a result, very complex yet unsatisfactory methods had been used in determining the noise factor NF of the preamplifiers.
So far the dominant noise was the so-called modulation noise from a system including a magnetic head and a magnetic tape. The lower the level of the reproduced signal, the less the modulation noise becomes as the results of the extensive studies and experiments conducted by the inventor prove. Since the modulation noise contains many amplitude modulated components, the modulation noise may be suppressed to some extent by passing the signal through a limiter prior to the frequency modulation.
Therefore the inventors made extensive studies and experiments in order to clarify the source of noise which greatly influences the signal-to-noise ratio S/N in the signal frequency demodulated from the signal reproduced from the magnetic tape upon which the signals have been recorded at a high density, and the inventors have succeeded in finding out the cause of noise and means for eliminating it or suppressing it as much as possible, to an extent hitherto unattainable by conventional preamplifiers.